This invention relates to a method of manufacturing a porous membrane and, more particularly, to a method of manufacturing a porous membrane from a vinylidene fluoride polymer.
Vinylidene fluoride polymer is excellent in mechanical strength and also in chemical stability with acid, alkali and organic solvents, and its membrane can find extensive applications as separable membranes such as filters in the fields of medical treatment. Such filters are effective for filtering bacteria and other small particles introduced into an infusion solution in a solution administration line.
To this end, a porous membrane with a uniform pore diameter is sought. Throughout the specification, by the term "vinylidene fluoride polymer" are meant vinylidene fluoride in situ, copolymers of vinylidene fluoride with ethylene tetrafluoride and propylene hexafluoride and mixtures of these compounds.
The porous membrane of vinylidene fluoride polymer can be manufactured by dissolving the polymer in a good solvent and then causing solidification of the resultant solution in a non-solvent.
At this time, a porous membrane having a desired pore diameter can be obtained by appropriately selecting the good solvent and non-solvent.
By the term "non-solvent" is meant a substance which in situ hardly dissolves polymer. By the term "good solvent" is meant a substance which can dissolve polymer in a temperature range below the melting point of the polymer. The non-solvent and good solvent have affinity to each other and can be mixed with each other.
Examples of the good solvent are acetone, dimethylformamide, dimethylacetoamide, dimethylsulfoxide and methylethylketone. These solvents are very excellent in solubility.
Examples of the non-solvent are 1,1,2-trichloro-1,2,2-trifluoroethane, dichloromethane, 1,1,1-trichloroethane, trichloroethylene, methanol, and isopropylalchol.
These good solvents, however, have high affinity to polymer. Therefore, solidification of the resultant solution in the non-solvent proceeds with difficulty and, as a result, a porous membrane lacking in the uniformity of the pore diameter is produced.
To solve this problem, it is attempted to add such additives as glycerine and surface active agent to the polymer solution for uniformalizing the pore diameter as disclosed in Japanese Patent Disclosures 58-98105 and 61-38207. However, the addition of additives to the non-solvent makes the process control difficult.
According to the Japanese Patent Disclosure 54-16382, acetone is used as good solvent. Acetone does not provide sufficient dissolving power at normal temperature.
When heated, however, it provides its intrinsic good-solvent property. Acetone has low affinity to polymer, so that it is possible to obtain a porous membrane having uniform pore diameter. However, the polymer solution has low stability and undergoes gellation in a short period of time after dissolution. It is thus difficult to obtain a smooth and uniform porous membrane, thus complicating the process.
To obtain a porous membrane having uniform pore diameter distribution from vinylidene fluoride polymer, it is necessary to dissolve polymer in a solvent, which has comparatively low affinity to polymer and hence provides insufficient dissolving power at normal temperature. In order to facilitate the process of manufacturing the membrane, it is important that the resultant polymer solution is held in a stable state free from gellation for a while even when it is cooled down to room temperature after the dissolution.